TamperMonkey script to detect Varnish hits

gistfile1.js

// ==UserScript==
// @name       Varnish Header Checker
// @namespace  http://www.thingy-ma-jig.co.uk/
// @version    0.1
// @description  Check for the presence of a Varnish Header and put an alert DIV on the page.
// @match      http://*/*
// @copyright  2014+, You
// ==/UserScript==

var req = new XMLHttpRequest();
req.open('GET', document.location, false);
req.send(null);
var headers = req.getAllResponseHeaders().toLowerCase();
console.log(headers);

var baseCSS = 'position:fixed;opacity:0.5;top:32px;left:32px;width:128px;height:32px;line-height:32px;padding:4px;z-index:999';

if (headers.indexOf('x-varnish-cache: miss') != -1) {
    // create almost any element this way...
    var el = document.createElement("div");
    // add some text to the element...
    el.innerHTML = "VARNISH MISS";
    el.style.cssText = baseCSS + ';border:1px solid #800;background-color:#f88;color:#FFF;';
    // "document.body" can be any element on the page.
    document.body.appendChild(el);
}
else if (headers.indexOf('x-varnish-cache: hit') != -1) {
    // create almost any element this way...
    var el = document.createElement("div");
    // add some text to the element...
    el.innerHTML = "VARNISH HIT";
    el.style.cssText = baseCSS + ';border:1px solid #080;background:#8f8;color:#000;';
    // "document.body" can be any element on the page.
    document.body.appendChild(el);
}